Method of manufacturing a coaxial electrical transmission line



Oct. 31, 1967 SEWELL 3 349,479

D. N. METHOD OF MANUFACTURING A COAXIAL ELECTRICAL TRANSMISSION LINEOriginal Filed Feb. 11, 1965 3 Sheets-Sheet 1 D. N. S EWELL.

Oct. 31, 1967 7 3,349,479

METHOD OF MANUFACTURING A GOAXIAL ELECTRICAL TRANSMISSION LINE 5Sheets-Sheet 2 Original Filed Feb. 11, 1965 Oct. 31, 1967 D. N. SEWELL3,349,479

METHOD OF MANUFACTURING A COAXIAL ELECTRICAL TRANSMISSION LINE OriginalFiled Feb. 11, 1965 3 Sheets-Sheet a V l A I I w ll uJL I! 'II I UnitedStates Patent 5 Claims. (Cl. 29600) ABSTRACT OF THE DISCLOSURE A coaxialRF transmission line has tubular inner and outer conductors. The innerconductor has a series of spaced annular grooves formed by forcing itsmetal into grooved mandrels inside the conductor by conjoint radial andaxial forces produced by a hydraulic ram and subsequent groove shapingby a rolling operation so that the outer surface of the inner conductoris smooth and continuous. Teflon disc insulators, each having anuninterrupted outer periphery and an inner periphery interrupted at fourpoints by radially extending slits, are slid axially along the innerconductor and positioned in the corresponding grooves. The innerconductor-disc assembly is then inserted into the outer conductor sothat the discs support the two conductors in coaxial relation.

This application is a divisional application of my copending applicationSer. No. 431,829, filed Feb. 11, 1965.

This invention relates to methods and apparatus for manufacturingelectrical transmission lines of the coaxial type.

A coaxial transmission line includes an outer conductor, typically acylindrical tube, and an inner conductor coaxially supported within theouter conductor, and this invention relates to the type of coaxialtransmission line in which the inner conductor is also tubular. It isessential to maintain the coaxial relationship of the two conductors inthe transmission line and for this purpose dielectric supports arespaced at intervals along the length of the transmission line tomaintain the tube conductors in position. Such transmission lines arewell-known and a conventional positioning support for the dielectricmember is a machined bushing of electrical conductive material, usuallyof the same conductivity as the material of the inner conductor. Thebushing is contoured to receive the dielectric member, typically a disc,and each end of the bushing is machined so that it will fit securelyinside the ends of adjacent sections of inner conductor tubing. Thetubing which is to serve as the inner conductor is then cut to specifiedlengths as determined by mechanical support and electrical performanceconsiderations. Each disc requires a bushing, and two soldered or weldedjoints are required for securing the respective ends of the tubing toeach bushing. The bushings, which form an electrical portion of theelectric circuit of the transmission line, must be accurately made andsmoothly finished in view of the radio frequency fields to which theyare exposed. The machining operations are expensive in view of theprecision required and also result in the waste of material. Further,the tubing must be cut into short lengths and then soldered or weldedtogether again to the bushings to form the total length of innerconductor, further increasing the cost of the assembly.

Another problem that arises in connection with such transmission linesrelates to the disc support. Conventionally, the disc is cut throughboth its inner and outer peripheries so that it may be spread apart andfitted onto the inner conductor with its inner periphery seated in thegroove in the bushing. The insulator, which is principally performing amechanical function by supporting a significant amount of weight butalso affects the electrical characteristics of the line, often must bedistorted substantially in order to place it onto the inner conductor,and this distortion results in permanent deformation of the disc whichmodifies the dielectric characteristics of the transmission line. Thedegree of permanent distortion varies widely and, therefore, isdiflicult to compensate for in the design of the transmission line.Also, the disc may not seat properly in the groove or the positioning ofthe inner conductor within the outer conductor may create frictionforces against the distorted outer periphery of the disc which tend topartially draw it out of the groove in the bushing in which it ispositioned. While the groove in the bushing is frequently modified toprovide better mechanical support for the insulator and reduce thistendency to pull the disc out on assembly, such modification degradesthe electrical characteristics of the transmission line. Due to thenature of the transmission line, it is very difficult to determine theposition of the dielectric disc after assembly, and therefore, theassembly of quality coaxial transmission lines of this type requires ahigh degree of care and skill.

Accordingly, it is an object of this invention to provide a novel andimproved method of manufacturing a coaxial transmission line.

Still another object of the invention is to provide a novel and improvedmethod of manufacturing a coaxial transmission line configurationsupport for a disc insulator.

Still another object of the invention is to provide novel and improvedmethods for manufacturing coaxial transmission line components.

The coaxial transmission line constructed in accordance with theinvention includes a tubular outer conductor and a tubular innerconductor disposed coaxially within the outer conductor. The tubularinner conductor has a continuous wall of substantially uniform thicknessand along the length of the inner conductor annular depressions areformed at spaced intervals. Each annular depression, formed without anyinterruption in the continuity of the inner conductor, receives adielectric support disc, which disc preferably has an uninterruptedouter periphery and an inner periphery that is interrupted at aplurality of points. Interruptions in the disc material extend towardthe outer periphery radially, spirally, or otherwise as desired. Eachdisc may he slid axially along the tubular inner conductor such that thedistortion effects are principally produced at points between the innerand outer peripheries, and its outer periphery is subjected tosubstantially no permanent distortion. The portion of the disc adjacentits inner periphery is seated in the groove so that it returns tosubstantially its original configuration and provides uniformcircumferential support for the inner conductor.

To form each annual groove a mandrel having an annular recess isinserted inside the inner conductor tube and aligned with a tubedeforming apparatus. The deforming apparatus is then actuated and exertsinward pressure uniformly around the tube and forces tube material intothe annular recess in the mandrel. During this application of pressureone end of the inner conductor is fed toward the mandrel so that theannular depression in the tube is formed without any substantialthinning of the tube wall. This depression then may be further shaped ina rolling operation to provide accurately defined support walls and aseating surface for the disc insulator. The tubing is subjected to onlymoderate stress during these operations, and no heating, soldering orwelding of the conductor is required so that there is no significantimpairment of its electrical characteristics. Also, there are no seamsor burrs in the conductor, but rather a smooth surface which enhancesthe mechanical and electrical capabilities and performance of thecoaxial line. Further, there are no foreign contaminants, such assoldering fluxes, introduced into the interior of the line which mayaffect its performance. The tubing after it is formed with thesegrooves, is cut to the precise length desired for the ultimate length ofcoaxial line and therefore the number of cutting operations and thetolerances for such cutting operations are reduced.

Other objects, features and advantages of the invention will be seen asthe following description of a particular embodiment thereof progresses,in conjunction with the drawings, in which:

FIG. 1 is a side elevational view, partially in section, of a coaxialtransmission line constructed in accordance with the invention;

FIG. 2 is a perspective view of a disc insulator utilized in thetransmission line shown in FIG. 1;

FIG. 3 is a perspective view of a method of assembly of the discinsulator on the inner conductor of the coaxial line;

FIG. 4 is a view, partially in section, of the disc insulator andtubular inner conductor in assembled relationship;

FIG. 5 is a diagrammatic view of apparatus for forming the annulargroove in the tubular inner conductor;

FIG. 6 is a view of the pressure applying apparatus for deforming thetubular inner conductor;

FIG. 7 is asectional view showing the relation of the pressure applyingapparatus, mandrel and inner conductor prior to deformation;

' FIG. 8 is a second view similar to FIG. 7 showing the positions ofapparatus when the tube is being deformed; I FIG. 9 is a top plan viewof the rolling apparatus used for modifying the groove formed by theapparatus shown in FIGS. 5-7,;

FIGS. 10, 11, and 12 are sectional views taken along the lines 10-10,11-11, and 12 12, respectively, of

FIG. 9; and

FIG. 13 is a sectional view similar to FIG. 12 showing the final formingof the groove in the inner conductor by the contoured rolls of theapparatus shown in FIGS. 9-12.

With reference to FIG. 1 there is shown a length of coaxial transmissionline having a tubular outer conductor 10 and a tubular inner conductor12. End flanges 14, 16 of conventional configuration are employed forsecuring lengths of coaxial line together, and the inner conductor has ajoint assembly 18 which is adapted to receive the end of a cooperatingtransmission line portion such as indicated at 20. The inner conductor12 is supported in spaced relation from the outer conductor 10 by aseries of disc insulators 22 of configuration that is indicated in ,FIG.2.

Each disc 22 is made of a suitable dielectric material such aspolytetrafluoroethylene (Teflon) which has a continuous outer peripheralsurface 24 and an inner peripheral surface 26 that is interrupted alongfour lines 28.

Each disc 22 is received in an annular groove 34 which has a baseportion 36 which supports the inner peripheral surface 26 of the discand two opposed side wall portions 38, 40 which provide axialpositioning and support for the disc as shown in FIGS. 1 and 4. Thesediscs are disposed in grooves spaced along the length of the innerconductor 12 at distances so that the discs will provide adequatemechanical support for the inner conductor 12 relative to the outerconductor 10. It will be noted that at each groove 34 there ispositioned within the inner conductor 12 a mandrel 42 which has a recess44 having side walls which define surfaces 38 and 40 of the annulargroove 34. The bottom surface member 36 of the groove 34 is normallyspaced from the bottom of the recess in the mandrel 42.

Each disc 22 may be positioned on the inner conductor by the methodindicated in FIG. 3 in that the disc is slid in the axial directionalong tubing 12 as displacement of the inner peripheral surfaces ispermitted by the plurality of outwardly extending interruptions 28without distorting the outer peripheral surface 24. Upon reaching theannular groove 34, the inner peripheral portion of the disc insulatorslips into the groove and the disc insulator is adjusted to the positionshown in FIG. 4 so that its major dimension extends perpendicularly tothe inner conductor 12. In this position the smooth and continuous outerperiphery 24 of the insulator is substantially undistorted and thereforeprovides a smooth mating surface for cooperation with the inner surfaceof the outer conductor 10 so that the inner conductor with disc supports22 secured in place may be slid into the outer conductor to the positionshown in FIG. 1 without substantial significant distortion of the discs.Prior to the coaxial assembly of the two tubes, the fittings of flangesand coupling as desired may be completed.

Apparatus for forming the annular groove in the inner conductor isindicated in diagrammatic form in FIG. 5. With reference to that figure,there is provided tube reducing apparatus including a hydraulic ramstructure 50 which acts on a reducing member disposed within collar 52.An end plate 54 is secured to the base member 56 which supports ramstructure 50, and pressure transmitting guides 58, 60' are disposedbetween the collar structure 52, the hydraulic ram structure 50, and theend plate structure 54, respectively. Suitable supports 62 are providedfor the tubular inner conductor 12 and a guide rod 64 having a threadedend to which a mandrel 42 may be secured enables a mandrel to bepositioned within the tube 12 in precise alignment with collar 52.

Additional details of this structure may be had with reference to FIGS.6-8. The hydraulic ram structure 50 includes cylinder 70 having a hollowram 72 through which the tube 12 passes. Secured to end plate 54 at oneend of the apparatus is a pneumatically operated tube grip generallyindicated at 74, and at the opposite end of the apparatus is a similartype tube grip 76 which is mounted for movement with the ram 72 and isconnected to that ram by tie rods 78. The collar 52 has a resilient ring8 0 secured within it and collar 52 acts to limit the radial movement ofthe resilient ring 80 outwardly. Positioned between the resilient ring80 and the ram head 72 are a pair of half-shell dies 82, 84 which makeup guide 60 and fit relatively closely over the outer surface of thetube 12 and couple the deforming force to ring 80 while guiding the tubeduring the deforming operation. A similar set of half-shell dies 86, 88which make up guide 58 is positioned between the resilient ring 80 andthe end plate 54. The ends of these dies are received within close fitbores indicated at 90, but those control bores are limited in length tofacilitate the disassembling of the half-shell dies after the ram isretracted to permit the tubing 12 to be moved from one station to thenext after formation of the groove 34. Tie rods 92 couple the end plate54 and the remote end of the hydraulic ram apparatus together.

In 'operation, the tube 12, is first inserted in the apparatus throughthe collar 52 as indicated in FIGS. 5 and 6. The half-shell dies 82, 84,86, 88 are then positioned in place to snugly fit over the tube. Mandrel42, secured to rod 64, is then inserted in the tube and preciselypositioned relative to ring 80 'by appropriate control means. Thehydraulic ram apparatus 50 is then actuated and moves the ram 72 towardsthe end plate 54 (towards the position indicated in FIG. 8). Thismovement acts to compress the resilient ring 80 and causes it to bulgeinwardly as indicated in FIG. 8 and deform the relatively soft coppertube into a depressed configuration in alignment with the recess in themandrel as indicated in FIG. 8. At the same time the hydraulic ram isacting through tie rod 78 on grip 76 to feed that portion of the tube 12axially so that tube material is being fed at the same time that thedepression is being formed. In a typical groove forming operation, thetravel of the ram 72 is in the order of one quarter inch and the lengthof tube 12 is reduced about one tenth of an inch. This conjoint feedingaction substantially eliminates any tendency to reduce the wallthickness of the tubing as the depression is being formed.

Through adjustment of the magnitude of force applied by traveling grip76 and the timing of the application of that grippin force, excellentcontrol over the degree of wall thickness reduction can be obtained.After the depression has been formed by this apparatus, the hydraulicram is retracted, permitting the tube 12 to be moved within the reducingapparatus; the positioning rod 64 is unscrewed from mandrel 42, removedfrom the machine and reloaded with another mandrel; and the apparatus isreset in position for the next reduction operation.

Mandrel 42 is made of brass which is easily machined to provide smoothlycurved surfaces 94 at the entrance of recess 44. In addition, the outercorners 96 are also curved to facilitate its insertion into the tube 12.It will be noted that the machining operation required for forming thismandrel is simple and does not require great precision.

Usually the groove formed by the reducing apparatus shown in FIGS. 5-8is not sufiicient, and therefore, a rolling operation to further formthe groove is performed by the apparatus shown in FIGS. 9-12. Thisapparatus employs three contoured hardened steel rolls 100, 102, 104.Each roll has a ridge portion 106 which, as best shown in FIG. 12, has aflat base 108 and two vertical side walls 110, 112, and control surfaces114. Rolls 100 and 102 are rotatably mounted on arm structure 116, Whileroll 104 is rotatably mounted on similar arm structure 118. The two armstructures are pivotally attached to a dual member link structure 120 sothat the three rolls may be arranged in triangular relationship relativeto the tubing 12 when it is positioned therebetween.

In operation of this rolling apparatus, the tube 12 with the annulardepression formed therein is positioned relative to the rollingstructure as shown in FIG. 12. The arms 116, 118 are then moved towardsone another so that the ridges 106 move into the depression. Arm members116 and 118 are forced towards one another as the structure is rotatedrelative to the tube. This rolling operation further deforms the metalin the depression into a groove havin the vertical side walls 38, 40 andbase 36 of limited depth so that the stress on the metal during theforming of the groove 34 is minimized. The final position of thecontoured rolls in the groove forming operation is indicated in FIG. 13with control surfaces 114 engaging the outer surface of tube 12. Thisoperation can be performed by one man on the tubing after the mandrelshave initially been secured in place and the depressions formed.

After the grooves have been formed in this rolling op eration, theTeflon disc 22 is slid axially along the inner conductor until its innerperiphery enters the groove 34 and then is positioned as shown in FIG. 4so that its inner peripheral surface is seated in the groove 34. Thesuitable couplings or end structures are assembled on the coaxialtransmission line structure and the inner conductor12 with the severaldiscs 22 positioned on it is slid into the outer conductor 10 to theposition shown in FIG. 1 to complete the assembly as indicated above.

While a particular embodiment of the invention has been shown anddescribed, it is recognized that many modifications will occur to thoseof ordinary skill in the art, and therefore, it is not intended that theinvention be limited to the disclosed embodiment or to details thereofand departures may be made therefrom within the spirit and scope of theinvention as defined in the claims.

What is claimed is:

1. In the manufacture of a coaxial electrical transmission line, thesteps of clamping an elongated tubular conductor component at twoaxially spaced points, positioning a mandrel having a support surface ofconfiguration substantially the same as the configuration of saidelongated tubular conductor component of the transmission line insupporting relation to said conductor component, said support surfacehaving an annular recess therein,

applying pressure in a radial direction to said tubular conductorcomponent at the point at which said mandrel is positioned to force themetal in said conductor component into the recess in said mandrel toform an annular groove in said conductor component while maintaining thesmooth continuity of the surface of said conductor component,

and moving one of said clamping points toward the other clamping pointto apply force to said conductor component in the direction of itslength during the application of said pressure to said conductorcomponent to reduce the length of said conductor component so that saidannular groove is formed without any substantial reduction in thethickness of the wall of said conductor component at that point.

2. The method as claimed in claim 1 wherein said conductor component isof circular cross-sectional configuration, and further including thefurther step of forcing the metal of said conductor groove into therecess in said mandrel in a rolling operation.

3. The method as claimed in claim 2 and further including the steps ofdisposing a dielectric disc member in said groove, and

inserting said conductor component and said disc member into a secondconductor component so that said first and second conductor componentsare supported in coaxial relation by said disc member.

4. In the manufacture of a coaxial electrical transmission line, thesteps of clamping a tubular conductor component at two axially spacedpoints,

positioning a cylindrical mandrel of diameter substantially the same asthe inner diameter of said tubular conductor component inside saidcomponent between said two spaced points to support an annular portionof said conductor component, the surface of said cylindrical mandrelhaving an annular recess therein, positioning a resilient disc memberover said conductor component in alignment with said annular recess,

applying pressure to said disc member to force the metal in the tubewall of said conductor component into the recess in said mandrel to forman annular groove in said conductor component, and

moving one of said clamping points toward the other during theapplication of said radial pressure to said tubular conductor to reducethe length of said conductor component so that said conductor metal isforced into said recess without any substantial reduction in the wallthickness of said conductor component.

5. The method as claimed in claim 4 and further including the steps ofdisposing a dielectric disc member in said groove, and

inserting said conductor component and said disc member into a secondconductor component so that said first and second conductor componentsare supported in coaxial relation by said disc member.

References Cited UNITED STATES PATENTS 3 Green 33396 Peck et a1 29148.4Crockett 29-5 16 Schuster 855 Looker 29-520X Ketchum 29-52 O X THOMAS H.EAGER, Primary Examiner.

1. IN THE MANUFACTURE OF A COAXIAL ELECTRICAL TRANSMISSION LINE, THESTEPS OF CLAMPING AN ELONGATED TUBULAR CONDUCTOR COMPONENT AT TWOAXIALLY SPACED POINTS, POSITIONING A MANDREL HAVING A SUPPORT SURFACE OFCONFIGURATION SUBSTANTIALLY THE SAME AS THE CONFIGURATION OF SAIDELONGATED TUBULAR CONDUCTOR COMPONENT OF THE TRANSMISSION LINE INSUPPORTING RELATION TO SAID CONDUCTOR COMPONENT, SAID SUPPORT SURFACEHAVING AN ANNULAR RECESS THEREIN, APPLYING PRESSURE IN A RADIALDIRECTION TO SAID TUBULAR CONDUCTOR COMPONENT AT THE POINT AT WHICH SAIDMANDREL IS POSITIONED TO FORCE THE METAL IN SAID CONDUCTOR COMPONENTINTO THE RECESS IN SAID MANDREL TO FORM AN ANNULAR GROOVE IN SAIDCONDUCTOR COMPONENT WHILE MAINTAINING THE SMOOTH CONTINUITY OF THESURFACE OF SAID CONDUCTOR COMPONENT, AND MOVING ONE OF SAID CLAMPINGPOINTS TOWARD THE OTHER CLAMPING POINT TO APPLY FORCE TO SAID CONDUCTORCOMPONENT IN THE DIRECTION OF ITS LENGTH DURING THE APPLICATION OF SAIDPRESSURE TO SAID CONDUCTOR COMPONENT TO REDUCE THE LENGTH OF SAIDCONDUCTOR COMPONENT SO THAT SAID ANNULAR GROOVE IS FORMED WITHOUT ANYSUBSTANTIAL REDUCTION IN THE THICKESS OF THE WALL OF SAID CONDUCTORCOMPONENT AT THAT POINT.